Question

I am trying to write a chip8 emulator in C#. It will be necessary to simulate in software the operations that take place in hardware on the real chip.

There is an opcode that requires detecting a whether a borrow occurred during the subtraction of two binary numbers.

Byte1 - Byte2

Does anyone have any ideas as to how I can use C# to tell if a borrow occurred or not?

Answer 1

You will need to compare each bit of the bytes… If the second byte bit is set but not the first, you have a borrow condition:

static void Main(string[] args) {
    byte b1 = byte.Parse(args[0]);
    byte b2 = byte.Parse(args[1]);

    bool borrow = false;
    for (int mask = 0x01; mask <= 0x80; mask <<= 1) {
        if ((b2 & mask) > (b1 & mask)) {
            borrow = true;
        }
    }

    Console.WriteLine(Convert.ToString(b1, 2).PadLeft(8, '0'));
    Console.WriteLine(Convert.ToString(b2, 2).PadLeft(8, '0'));
    Console.WriteLine("borrowed: {0}", borrow);
}

There may be a very clever boolean logic that gives you the answer, but I cannot come up with it now.